Hydrocyclones are commonly used in many industries for carrying out concentrating, clarifying and classifying operations on various mineral slurries or pulps and liquids containing undissolved solid particles. Briefly, a hydrocyclone consists of a separating chamber that is annular in section and which has a conical portion having an underflow discharge opening at its lower apex end. The chamber also has means forming an inlet opening connected tangentially with the upper portion of the chamber and means forming an overflow outlet which communicates with a vortex finder disposed axially within the chamber. When in operation, the feed is supplied under pressure to the inlet, and swirling movement of the body of material within the chamber causes centrifugal separation of solids with the separated material being discharged as an underflow from the apex end of the chamber, and the overflow being discharged through the vortex finder and the overflow outlet. For concentrating or where it is desired to provide a clarified overflow, the operation is such that substantially all of the solid material of the feed is discharged with the underflow. For classification, heavier solids are discharged in the underflow and lighter solids in the overflow.
A common problem in the operation of hydrocyclones has been the maintenance of a constant high density (solids-to-liquid ratio) underflow material while operating under conditions where the cyclone feed density fluctuates over wide limits. Such fluctuations are experienced for example in mineral slurries produced by continuously operating product preparation circuits. By way of example, in instances where a sand-gravel preparation circuit is supplying feed to a hydrocyclone, the density of the feed may vary from less than 1% to more than 25%, with the result that the underflow is subject to corresponding fluctuations in density. Such variations may cause serious resulting problems in the handling and further processing of the underflow.
In the past various methods and types of equipment have been employed in an effort to control the density of the underflow. For example, in some instances variations in density of the underflow have been detected by various devices, such as those making use of gamma ray radiation that is projected through a stream of the material, and the detecting device connected to control the circuit which is preparing the feed. Such equipment is relatively expensive and the control provided is not as accurate as is frequently desired, due to deficiencies in the detecting devices, inability to effectively control the preparation circuit, or both. Less elaborate devices that have been employed include collapsible tubing of resilient material, flap valves, and counterbalanced piping arrangements applied to the apex of the hydrocyclone to effect some control over the discharge of underflow in accordance with changes in density. Use of such devices has resulted in increased maintenance requirements of the hydrocyclone circuit, cyclone choking or plugging, and aberrant performance. In addition, such devices do not provide maintenance of the underflow density within the flow limits frequently desired.